Field of the Invention
The present invention generally relates to video cameras and, more specifically, to a video camera using a solid state image pickup element such as a charge-coupled device (CCD) or the like whose storage time can be changed so as to change the shutter speed of the camera.
Description of the Prior Art
In recent years, it has been proposed in a video camera using a solid state image pickup element such as a charge-coupled device (CCD) to control the shutter effect or speed of the camera by controlling the storage time of the CCD, as disclosed in Japanese Patent Laid-Open Publication (JP-A) No. 64-77378, for example.
It has also been investigated to make it possible to arbitrarily set the shutter speed, for example, so as to realize various effects of photographing.
When setting the shutter speed arbitrarily in the conventional video camera, the storage time of the solid state image pickup element is changed arbitrarily, so that the output signal level of the solid state image pickup element is also changed. Thus, if the shutter speed is changed from a maximum speed of one thousandth of a second (1/1000 sec.) to a target speed such as a normal speed of one sixtieth of a second (1/60 sec.) as shown in FIG. 1A, the output signal level of the image pickup element changes abruptly as shown in FIG. 1B to thereby cause distortion temporarily in the video signal such as saturation or lack of sufficient level.
In the video camera, an automatic gain control (AGC) amplifier is generally provided at the output side of the CCD. The AGC amplifier is generally provided with a time constant of several seconds in order to eliminate the distortion of the output signal level of the CCD due to noise and the like, so that the AGC amplifier can not follow the output signal level of the CCD when the shutter speed is changed abruptly.